Slotted coupling device for waveguide



United States Patent() Rabindra Nath Ghose, Los Angeles, Calif., and Anatole Turecki, Haddoneld, NJ., assignors to Radio Corpou ration of America, a corporation of Delaware Filed time 21, 1951, ser. N0. 667,235

2 Claims. (c1. 34e-.767) Y The invention relates to a coupling device, and particularly to a coupling device for slot antennas or slotted waveguides. An object of the invention is to provide an improved coupling device for use with a slot antenna. Another object of the invention is to provide an improved coupling device for use with slotted waveguides. Another object of the invention is to provide an improved coupling `device that can be used with a slot antenna, and that permits the phase and amplitude of the radio' frequency energy radiated by the slot antenna to be changed easily and independently.

Another object is to obtain independent control of the amplitude and phase of the energy fed across a slot radiator, and yet maintain a symmetrical voltage distribution about the axis of the slot. Another object is to provide a novel coupling device for an ultra high frequency (UHF) antenna system capable of handling a large power; for example, of the order of ve megawatts of .effective radiated power. T hecoup1ing device of the invention is an electric coupling typeof feed designed to be used with a hollow waveguide .or a coaxial transmission line having a slot, andvprovides improved means for coupling radio frequency Venergy between the interior of the waveguide and the external surrounding space. As used herein, the term waveguide may include a coaxial transmission line.A Such a waveguide or line is frequently used as a slot antenna. Briefly, the coupling device comprises at leastv two probes which are attached to opposite edges of the slot at points removed from the middle or center of the slot. It is preferred, though not necessary, that the points at which the probes are attached be spaced from the .rn iddle or center by equal amounts. The two probes have different lengths and extend different distances from their points of attachment at the edges of the slot into the interior of thewaveguide or line. Each of the probes develops a voltage in response to radio frequency energy in the waveguide, and since the probes have different lengths, the magnitudes of the two voltages developed arev also diierent. The difference in the magnitudes of the two voltages develops a resultant voltage across the slot, and this resultant voltage couples radio frequency energy between the interior of the waveguide or line and the external surrounidng space. The amount of the radio frequency lenergy so coupled is determined, in part, by the difference in the lengths of the probes. The phase of the radio frequency energy so coupled is determined, in part, by the inherent capacities of the probes, and these capacities are determined by the actual lengths of the probes. Since the lengths of the probes can be independently varied, considerable flexibility of the coupling system is obtained because the amplitude and phase of the coupled radio frequency energy can be easily and independently varied. This feature is quite desirable in a high frequency, high-gain slot antenna having a plurality of slots arranged to provide a predetermined radiatin pattern. In other embodiments of the invention, a

i I'2,948,895 Patented Aug. 9, 1960 ICC plurality of pairs of probes are attached to the edges of the slot, the probes of each pair being attached to opposite edges of the slot at points removed from the center of the slot.

The invention is explained in detail in connection with the accompanying drawing, in which:

Fig. 1 shows a longitudinal view of a slotted coaxial transmission line utilizing the invention;

Fig. 2 shows a transverse cross-sectional view taken along the lines 2 2 in Fig. 1;

Fig. 3 shows, in a general way, an equivalent electrical circuit of the arrangement shown in Figs. 1 and 2;

Fig. 4 shows a transverse cross-sectional view of a coaxial transmission line utilizing probes having a diiferent construction;

Fig. 5 shows a perspective view of a slotted coaxial transmission line utilizing another embodiment of the invention;

Fig. 6 shows a perspective View of a slotted, hollow rectangular waveguide utilizing the invention; and

Fig. 7 shows a portion of a slot antenna utilizing the invention.

Referring to Fig. l of the drawing, .there is shown a coaxial transmission line 10 having a hollow, cylindrical outer conductor y12, and a concentric cylindrical inner conductor 14 positioned within the outer conductor 12. While the coaxial line 10 comprises concentric inner and outer conductors 14, 12, the invention can also be used with hollow waveguides having no inner conductor, such as a hollow cylindrical or a hollow rectangular waveguide. The dominant mode for the coaxial line 10 is the TEM mode, and radio frequency energy propagated through the coaxial line 10 has a transverse electric field in the radial direction. A generally rectangularly shaped slot 16 is placed in the outer conductor 12 of the coaxial line 10. Other slot congurations may also be used. Instead of the slot 16 being positioned with its longer edges positioned substantially parallel to the longitudinal axis of the coaxial line 10 as shown, the slot 16 may be oriented in other positions, such as with its longer edges positioned at some angle with respect to the longitudinal axis of the coaxial line 10.

Without the use of a coupling device for the slot 16, practicallyno voltage is developed across the slot 16 of the coaxial line 10, as described so far, in response to the radio frequency energy in the TEM mode in the line 10, and very little or no radio frequency energy is coupled between the interior of the coaxial line 10 and the external surrounding space. However, in accordance with the invention, a voltage is developed across the slot 16 by means of two metallic, rod-like probes 18, 20' which are of unequal lengths and which are fastened to the opposite longer edges of the slot 16 at points removed from the middle or transverse center line of the slot 16. It is preferable, though not necessary, that the points of attachment be spaced equidistant from the middle or transverse center line of the slot 16 because such an arrangement provides a symmetry of voltage distribution along the longitudinal direction of the slot 16. Since the voltage distribution along the longitudinal direction of the slot 16 determines the radiation pattern of energy radiated by the slot 16, the determination of the amplitude and phase of energy at the slot 16 from the radiation pattern becomes complicated unless the voltage distribution is symmetrical about the middle or center line of the slot 16.' It is preferred that the points of attachment of the probes 18, 20 to the slot edges be spaced a quarter wavelength at the operating frequency of the transmission line 10 from the middle of the slot 16, as indicated on the drawing, because it is relatively easy to adjust the probes 18, 20 le); thlat they have the proper impedance match for the e 0. 1 Y

Since radio frequency energy in the TEM mode has an electric field that is substantially radial in the coaxial line 18, a voltage may be developed by the probes 18, 20 if the probes 18, 2li are positioned in the line 10' so that their longitudinal axes coincide as nearly as possible with the electric field in the line 10. For a given probe, the greatest voltage will be developed by the probe when it lies in a plane that is perpendicular to the longitudinal axis of the line and when it lies in a radial line of the line 10. In other positions, less voltage will be developed by the probe. From a practical standpoint, it may be more convenient to position the probes 18, 2l) so that their longitudinal axes are parallel instead of radial. With the orientation of the probes 18, 2t)` lixed, the amount of voltage developed by the probes 18, 201 is then dependent upon the length of each of the respective probes 18, The voltages induced in or developed by the probes 18, 20 appear at the edges of the slot 16, and the net voltage or the resultant Yvoltage across the edges of the slot 16 is determined by the dierence in magnitudes of the two voltages developed by the probes 18, 28. The resultant voltage across the edges of the slot 16 causes radio frequency energy to be radiated into the surrounding space outside the coaxial line 10, the amount of energy radiated being proportional to the magnitude of the resultant voltage. Thus, since the difference in lengths of the two probes 18, 28 can be made to vary from a very high value to a very low value (as by making the probes 18, 20 adjustable in length), a wide range of resultant voltage across the slot 16, and hence a wide range of radiated energy, can be obtained.

The length of each of the probes 18, 28 also determines the capacity present in or introduced by the respective probes 18, 201. Since the absolute lengths of the probes 18, 20 can be varied over a wide range, the capacities of the probes 18, 20, and hence the phase relation of the resultant voltage developed across the slot 16 with respect to the voltage in the coaxial line 10, can also be varied over -a wide range. As a result, the phase relation of the external radiated field with respect to the radio frequency iield in the interior of the coaxial line 10` can be varied over a wide range. In order that the lengths of the probes 18, 28 can be easily adjusted, they may be threaded at their ends for engagement in threaded holes in the outer conductor 12 of the line 10, in the manner of a threaded screw which is adjustable in effective length.

One equivalent electrical circuit of the arrangement shown in Figs. l and 2 is shown in Fig. 3. Each of the probes 18, 28 can be considered as being a series circuit comprising a voltage generator and a capacitor. The generators are designated G1 and G2 respectively, and the capacitors are designated C1 and C2 respectively.` The series circuits may be considered as being connected in parallel with each other and in parallel with the slot impedance and the radiation resistance. It will be seen that the resultant voltage across the slot impedance and radiation resistance is determined by the difference in voltage developed by the generators. resultant voltage, as already mentioned, is actually determined by the difference in lengths of the two probes 18, 20. Also, it will be seen that the phase of the voltage developed across the slot impedance and the radiation resistance is determined by the sum of the capacity present in or introduced by the probes 18, 20. As previously mentioned, the voltages developed by the probes 4and the capacities presented by the probes can be varied independently. in addition to the capacity introduced by the probes 18, Ztl in the slot, the capacitive reactance across the slot 16 due to the `asymmetrical transmission line formed by the probes 18, 28 also influences the phase of the radiated energy with respect to the radio frequency energy in the coaxial line 18. This reactance is shown as the capacitor C3 in Fig. '3. Thus, the three capacitors C1, C2 and C3, provide flexible control of the 4 v phase of the radiated energy without aecting the amplitude of the radiated energy.

Fig. 4 shows another embodiment of the invention. This embodiment is similar to the embodiment shown in Figs. l and 2 except that the physical construction of the probes 18, Ztl has been modified. In Fig. 4, each o-f the probes 22, 24 is provided at its free end with a metallic capacitive plate or disc 26, the function of which is to provide additional capacitance in the probes 22, 24. The plates or discs 26 can have any desired configuration, shape, or area. The plates or discs 26 give additional iiexibility to the coupling device in that their area, and hence their effective capacity, can be varied over a wider range than in the case of the probes 18, 20 shown in Figs. 1 and 2.

Fig. 5` shows another embodiment of the invention as applied to a coaxial line 30 having a hollow, cylindrical outer conductor 3-2, and a concentric cylindrical inner conductor 34 positioned within the outer conductor 32. A rect-angularly shaped slot 36 is placed in the outer conductor 32 of the coaxial line 30 so that the longitudinal axis of the slot 36 is parallel to the longitudinal axis of the coaxial line 30. Two pairs of probes 38, 40, 42, `rtl4 are fastened to the longer edges of theslot 36. The probes 38, 40, of the first pair of probes 38, 40 are fastened to opposite edges of the slot 36 at points substantially equidistant `from the middle or transverse center lline of the slot 36, and the probes 42, 44 of the second pair of probes 42, 44 lare fastened to opposite edges of the slot 36 at points substantially equidistant from the middle or transverse center line of the slot 36. However, the second pair of probes 42, 44 are on the opposite side of the middle of the slot 36 relative to the rst pair of probes 38, 40. Although not necessary, it is preferable that the probes of each pair of probes be spaced the same distance from the middle or transverse center line of the slot 36 because such an arrangement provides a symmetrical voltage ldistribution about the center of the slot 36 along the longitudinal direction of the slot. This symmetrical arrangement is desirable so as to simplify the design of an antenna which is constructed in accordance with the invention, and which must have a predetermined far-eld radiation pattern. This preferred spacing is la quarter wavelength at the operating frequency, as shown. One pair of probes 38, 40 are of unequal lengths, and similarly, the other pair of probes 42, 44 are also of unequal lengths, the shorter length probes 38, -44 lbeing fastened to opposite edges of the slot 36. Thus, two voltages are developed across the edges of the slot 36 instead of one as described in 'connection with Figs. 1 and 2. With two voltages being developed across the edges of the slot 36, a more flexible arrangement is obtained because each of the probes may be independently varied with respect to the other probes. This permits the phase and amplitude of the radio frequency energy coupled by the slot 36 to the surroundingV space to be varied fand controlled over a wide range.

While the coupling device has been shown in connection with coaxial transmission lines, it is to be understood that the coupling device can be used in any slotted waveguide having probes positioned parallel to the transverse electric eld, such as a hollow circular waveguide which has a dominant TEM mode with a non-zero radial electric field. Furthermore, the coupling device is also effective for use ina hollow rectangular waveguide which carries the dominant TEM, mode. For example, Fig. 6 shows ya rectangular, hollow waveguide 50 having a rectangular slot 52, and two pairs of probes S4, 56, S8, 60 fastened to the longer edges of the slot 52, the probes of each pair being `of different lengths as described in connection with Fig. 5. The probes 54, 56, 58, 60 function `in a manner similar to the probes shown in the coaxial transmission lines in the other gures of the drawing.

A coupling device in accordance with the invention is quite useful in connection with a high frequency slot antenna which is intended to r-adiate radio frequency energy through a plurality of slots. These slots may be positioned in various configurations around the antenna, and above one another in various arrays. A portion of a coaxial slot antenna 70 having pairs of probes in accordance with the invention is shown in Fig. 7. The probes provide independent control of the phase and amplitude of the voltage developed across each of the slots, and hence provide independent control of the phase and amount of radio lfrequency energy radiated by each of the slots. This independent control is not only desirable, but is actually necessary for a high lgain slot antenna which must satisfy certain coverage and directivity requirements. The depth of penetration of the probes inside the coaxial line may be made adjustable by providing threaded probes which may be screwed in and out of threaded holes placed at the edges of the slot. Such a feature may be quite useful in adjusting the radiation pattern of an antenna such as shown in Fig. 7. Since the probes develop a voltage that is applied to the slots, and this voltage is limited only by the break-down strength of the probes in the iield inside the hollow waveguide or coaxial line, the coupling device is capable of handling large amounts of power. The break-down voltage of the coupling device is limited only by the dimensions of each of the slots and the probes, and ordinarily these dimensions may -be so designed as to handle a large amount of power. And, as previously mentioned, it is possible to control the voltage .and phase of the radio frequency radiated by each of the slots independently, thus providing a slot antenna Whose directivity in both the horizontal and vertical directions can be closely controlled. The invention enables symmetrical radiation about the longitudinal axis of the slot, and also symmetrical radiation in both halves of the slot. An Iantenna. designed in accordance with the invention can be used to radiate either unidirectional or omnidirectional television signals over a wide band of frequencies.

What is claimed is:

1. A device -for coupling radio frequency energy between the interior of a waveguide and the surrounding space thru a slot in said waveguide, comprising a pair of probes attached to opposite edges of said slot at points which are on opposite sides of and removed from the respective midpoints of said edges of said slot by substantially one-quarter Wavelength at the mean operating frequency of the energy in the Wave guide, said probes extending dilerent distances from said points into said interior.

2. A coupling device for a hollow metallic conductor capable of carrying radio frequency energy and having a slot, comprising a pair of probes attached to opposite edges of said slot at points which are on opposite sides of and removed from the respective midpoints of said edges of said slot by substantially one-quarter of a wavelength at the mean operating frequency, said probes being positioned to extend from said points into the nterior of said waveguide, and said probes being adjustable and having different lengths relative to each other as measured between said points and the ends of said probes Within said interior.

References Cited in the le Aof this patent UNITED STATES PATENTS 2,574,433 Clapp Nov. 6, 1951 2,629,051 Lindenblad Feb. 17, 1953 

